1. Field of the Invention
This invention relates to multiline telephonic communication systems, and it relates, in particular, to the arrangement and use of a line interface unit (LIU) in such a system.
2. Description of the Prior Art
Prior workers have made various investigations into digital switching systems for the communication field. Examples of these include the G. D. Johnson et al. U.S. Pat. No. 3,736,381 which is directed to a digital toll telephone switching system. Hybrid space division and time division switching networks are also known as evidenced by the M. E. M. Bosonnet et al. U.S. Pat. No. 3,689,701, and a paper entitled "Exploratory Pulse Code Modulation Integrated Switching and Transmission System for Local Networks" by S. R. Treves et al., at pages 77 to 88 of Electrical Communication, Vol. 47, No. 2, 1972. In the hybrid systems a space division switch is utilized for concentrating analog call signals which are then encoded in digital form and multiplexed prior to application to a time division switching network. In addition, Treves et al. teach the use of the switching network for limited communication between the central processing unit and the concentrators.
Further work in the time division switching field is found in a paper "ESSEX -- A Continuing Research Experiment in Time--Separation Communication" by D. B. James et al., The Proceeding of the Institution of Electrical Engineers, Paper No. 3387E, November 1960, Vol, 107B, pages 330-335. One of the concepts utilized in the system described there is the employment of one bit per pulse code modulation word for unspecified functions and of one time slot per frame for supervisory functions.
A time division switch in a remote concentrator in which communication with a central office is provided by way of multiplexed supervisory and control signals in the call signal path is taught in the Inose et al. U.S. Pat. No. 3,492,435. The latter patentees also teach a line unit employed in their concentrator and in which there are performed functions such as 2-wire-to-4-wire conversion, analog-digital conversion, and multiplexing of coded call signals and a line status signal onto a common digital path. Inose et al. use low voltage tone ringing, rather than the high voltage low frequency ringing usually found in the public telephone network, to reach their line units. Their scanning system is used to initiate ring trip with the risk of false indications on lines with multiple ringers or of ring generator overload if scan sampling is limited to the no-ring interval.
Another paper, entitled "An Approach to the Implementation of Digital Filters" by L. B. Jackson et al., IEEE Transactions on Audio and Electroacoustics, Vol. Au-16, No. 3, Sept. 1968, pages 413-421, shows arrangements for time-sharing digital filters for limited purposes in a telephone switching office that could otherwise be either analog or digital.
It can thus be seen that digital switching offices have been tested in various forms to demonstrate the feasibility of time division switching. However a practical local office that is essentially digital from line unit inward has not been realized. It is suggested at page 35 in Telephony, July 17, 1972, in an article.
"SPC and TDS Dominate International Switching Meeting" by M. Hobbs, that the high cost of the digital local office has encouraged the hybrid office studies. Among the reasons are factors such as the need in a practical local, or end, office for a number of functions to be performed on a perline basis which usually calls for extensive distribution and control circuits, in addition to the regular call connection circuits, in the office. One example of such a function is ringing. A ringing relay at a line unit must be controlled, and ring-trip detection must be provided for high-level ringing signals that are widely used in the public telephone network but which cannot be switched through the electronic crosspoints frequently employed for time division switching. The cost of the distribution and control circuits for the ringing functions and for other functions, plus the cost of per-line analog-digital codec circuits, have heretofore made the digital local office a very costly project.